1. Combine the melted butter with the graham crackers in a small bowl, then divide the mixture evenly amongst 5-6 mini mason jars (4 oz each). The crust will be about 1″ high. 
2. Zest the key limes, then juice them all into a measuring cup. You will need 2 tbsp of the key lime juice and 2 teaspoons of zest.
3. In the bowl of a stand mixer fitted with the paddle attachment, or using an electric handheld mixer, whip the whipping cream for a minute or so, before adding in the powdered sugar. Whip on high for 3-4 minutes or until stiff peaks form. Set the whipped cream aside.
4. Whip softened cream cheese next, along with the key lime juice, for about 1 minutes. Using a spatula, fold in the whipped cream and lime zest until smooth. Divide the mixture evenly amongst the jars, on top of the graham cracker crust. 
5. Place the jars in the fridge to chill for at least 4 hours before serving. Top with extra whipped cream, key lime zest or graham crackers.

Leftovers will keep in the fridge for 3-4 days, covered.